During evaluation of bowel function, which reflex is specifically involved in relaxing the internal anal sphincter in response to rectal distension?

During evaluation of bowel function, which reflex is specifically involved in relaxing the internal anal sphincter in response to rectal distension?

• A. Gastrocolic reflex.
• B. Sampling reflex.
• C. Rectoanal inhibitory reflex.
• D. Anal wink reflex.

Answer: (C)

Rectal distension triggers a reflex that relaxes the internal anal sphincter, a mechanism that allows the rectum to sense stool and decide whether to pass it along to the anal canal. This is the rectoanal inhibitory reflex: as the rectum fills, afferent signals cause the smooth muscle of the internal anal sphincter to relax briefly, reducing anal resistance and enabling assessment of the stool before voluntary defecation. In healthy individuals, this relaxation helps prevent leakage while still allowing eventual defecation controlled by the external anal sphincter. The other options aren’t about this specific relaxation response: the gastrocolic reflex is about increased colonic motility after meals, not relaxation of the internal anal sphincter; the sampling reflex relates to sensing and sampling stool content to guide continence/defecation decisions but does not describe the targeted relaxation of the internal anal sphincter; the anal wink is a somatic reflex causing contraction of the external anal sphincter in response to stimulation, not relaxation of the internal sphincter.

Rectal distension triggers a reflex that relaxes the internal anal sphincter, a mechanism that allows the rectum to sense stool and decide whether to pass it along to the anal canal. This is the rectoanal inhibitory reflex: as the rectum fills, afferent signals cause the smooth muscle of the internal anal sphincter to relax briefly, reducing anal resistance and enabling assessment of the stool before voluntary defecation. In healthy individuals, this relaxation helps prevent leakage while still allowing eventual defecation controlled by the external anal sphincter.

The other options aren’t about this specific relaxation response: the gastrocolic reflex is about increased colonic motility after meals, not relaxation of the internal anal sphincter; the sampling reflex relates to sensing and sampling stool content to guide continence/defecation decisions but does not describe the targeted relaxation of the internal anal sphincter; the anal wink is a somatic reflex causing contraction of the external anal sphincter in response to stimulation, not relaxation of the internal sphincter.